Why is the screen saver not kicking in?

Solution 1:

I cannot tell about screensaver for sure, but you can use pmset -g to show which process is disturbing sleep (think this first appeared in 10.6.7)

# pmset -g
[cut]
sleep       0 (imposed by 1517)
[cut]

this means that pid 1517 is preventing the system from sleeping.

# ps ax |grep 1517
1517   ??  Rs     1:59.76 /System/Library/CoreServices/backupd 

This shows that this is the backup daemon running.

Solution 2:

I have seen this caused by some poorly behaved programs, notably Dungeon Crawl Stone Soup prevents the screen saver from working at all while it is running, whether full screen or windowed. You might record ps -ef from when it works and when it doesn't and look for any additional programs running that might be blocking the screen saver.

I've also seen some cheap mice develop "jitter" which makes the computer think it's being moved; this can also result in the machine waking up from suspend.

Solution 3:

Open up Activity Monitor and when you see the list of processes running, right click one of the column headings (e.x. "Process Name") and check "Preventing Sleep". This will add another column with "Yes" or "No" values per process. If you see one with "yes" in that column, it means that this process is what is keeping your mac from sleeping.